One way audio. I will bet a burger that the trouble is one of the A400 is behind a router, not on public I.P. address.
Write to email@example.com. I have a way to test from the outside wich one of the Quintums is not working O.K.
If you have a router in one of the ends, be sure that you either:
1) Put the a400 on the DMZ zone
2) Or be sure that all the VOIP ports are forwarded from the WAN I.P. to the inside I.P. where the quintum is at.
For a complete list of the ports that you need to forward, go to Quintum.com, and enter “customer service”